The invention relates to sulfinic acid derivatives and their preparation and use in various application areas.
As is known, sulfinic acid, H2SO2, is one of the strongest known reducing agents. The free sulfinic acid is unstable. Accordingly, it is only available commercially in the form of its stable and correspondingly manageable derivatives.
The following sulfinic acid derivatives have to date achieved economic importance:
1. Sodium dithionite (fiber bleaching in papermaking, vat dyeing and textile bleanching, mineral bleaching, heavy metal reduction in industrial wastewaters)
2. Sodium formaldehyde sulfoxylate dihydrate (textile discharge printing, textile bleaching, redox cocatalyst in emulsion polymerization, heave-metal reduction, pharmaceuticals)
3. Formamidinesulfinic acid (fiber bleaching in papermaking, textile bleaching)
4. Zinc formaldehyde sulfoxylate (textile printing and textile bleaching)
All of the above-mentioned sulfinic acid derivatives are use in the form of aqueous solutions of dispersions. In aqueous media, sodium dithionite and alkali metal formamidinesulfinatexe2x80x94the free formamidinesulfinic acid is virtually insoluble in water and, in its acid form, has only a very slight reducing actionxe2x80x94are only stable for a short time. As a result, even at room temperature they exhibit an excellent reductive capacity and an excellent bleaching effect on fibers. Aqueous preparations of sodium formaldehyde sulfoxylate and of zinc formaldehyde sulfoxylate are stable at room temperature for months. As a result, both formaldehyde sulfoxylates only exhibit their true reducing action at temperatures above 90xc2x0 Celsius. In strongly alkaline or acidic media or in the presence of suitably strong oxidizing agents, both formaldehyde sulfoxylates do of course also have a reducing effect at temperatures lower than 90xc2x0 C. This particular property of the formaldehyde sulfoxylates, namely to exhibit a very uniform and easily controlled reducing effect at temperatures between 5xc2x0 C. and 90xc2x0 C., is made use of in free-radical-initiated emulsion polymerization. Here, the formaldehyde sulfoxylates are used in various emulsion polymerization systems. In the case of the cold preparation of SBR (styrene butadiene rubber), the polymerization is initiated using organic peroxides. At the low polymerization temperature of about 5xc2x0 C., the organic peroxides do not, however, decompose into the required free radicals. The peroxide cleavage must be initiated by catalytic amounts of iron(II) salts. The iron in oxidation stage two is converted into oxidation stage three making it no longer suitable for the peroxide cleavage. With the help of the formaldehyde sulfoxylate, the iron(III) ions are again reduced to iron(II) ionsxe2x80x94the peroxide cleavage and the free-radical initiation continues. In other emulsion polymerization systems, peroxide compounds, such as hydrogen peroxide or peroxodisulfate, are used as free-radical formers. In order to increase the rate of free-radical formation, reducing agents are again used. Examples which may be mentioned are formaldehyde sulfoxylates, bisulfites, ascorbic acid, isoascorbic acid and sodium erythrobate. Formaldehyde sulfoxylates, in particular sodium formaldehyde sulfoxylate, have proven to be particularly effective and good value reducing agents. During the reduction process, however, the formaldehyde sulfoxylates eliminate formaldehyde. Plastics or polymer dispersions which must not contain formaldehyde are polymerized either using bisulfites, ascorbic acid, isoascorbic acid or sodium erythrobate. Since the formaldehyde-free reducing agents are weaker reducing agents, the disadvantage of less complete polymerization compared with formaldehyde sulfoxylates must be accepted. Furthermore, the use of ascorbic acid, isoascorbic acid and of sodium erythrobate leads to an undesired yellowing of the polymer.
The object of the present invention is to provide novel sulfinic acid derivatives whose chemical properties are as similar as possible to those of formaldehyde sulfoxylate, but which do not eliminate formaldehyde during or after use.
Surprisingly, it has now been found that this object is achieved by sulfinic acid derivatives of the type described in more detail below.
The present invention thus provides sulfinic acid compounds of the formula (I): 
where
M is a hydrogen atom, an ammonium ion, a monovalent metal ion or an equivalent of a divalent metal ion of the groups Ia, IIa, IIb, IVa or VIIIb of the Periodic Table of the Elements;
R1 is OH or NR4R5, where R4 and R5 independently of one another are H or C1-C6-alkyl;
R2 is H or an alkyl, alkenyl, cycloalkyl or aryl group, it being possible for these groups to have 1, 2 or 3 substituents which are chosen independently of one another from C1-C6-alkyl, OH, Oxe2x80x94C1-C6-alkyl, halogen and CF3; and
R3 is COOM, SO3M, COR4, CONR4R5 or COOR4, where M, R4 and R5 are as defined above, or, if R2 is aryl, which may be unsubstituted or substituted as defined above, R3 is also H,
and the salts thereof.
For the purposes of the present invention, the expressions listed below have the following meanings:
Alkyl represents straight-chain or branched alkyl groups which preferably have 1-6, in particular 1-4, carbon atoms. Examples of alkyl groups are methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl, n-hexyl, etc.
The same applies to the alkyl groups in O-alkyl.
Alkenyl represents straight-chain or branched alkenyl groups which preferably have 3-8 carbon atoms, in particular 3-6 carbon atoms. A preferred alkenyl group is the allyl group.
Cycloalkyl is, in particular, C3-C6-cycloalkyl, cyclopentyl and cyclohexyl being particularly preferred.
Aryl (also in aralkyl) is preferably phenyl or naphthyl. If the aryl radical is a phenyl group and is substituted, it preferably has two substituents. These are, in particular, in the 2- and/or 4-position.
Halogen represents F, Cl, Br and I, preferably Cl and Br.
M is preferably an ammonium ion, alkali metal ion or an equivalent of an alkaline earth metal ion or zinc ion. Suitable alkali metal ions are, in particular, sodium and potassium ions. Suitable alkaline earth metal ions are in particular magnesium and calcium ions.
R1 is preferably a hydroxyl or amino group.
R2 is preferably a hydrogen atom or an alkyl or aryl group which may be substituted as above. It preferably has one or two hydroxyl and/or alkoxy substituents.
R3 is preferably either COOM or COOR4 (M and R4 are as defined above) or, if R2 is aryl, which may be substituted as stated above, may also be a hydrogen atom.
A preferred embodiment covers compounds of the formula (I) in which
M is an alkali metal ion or an equivalent of an alkaline earth metal ion or zinc ion;
R1 is a hydroxyl or amino group; R2 is H or alkyl; and
R3 is COOM or COOR4, where M is H, an alkali metal ion or an equivalent of an alkaline earth metal ion, and R4 is C1-C6-alkyl.
A further preferred embodiment covers compounds of the formula (I), in which
M is an alkali metal ion or an equivalent of an alkaline earth metal ion or zinc ion;
R1 is a hydroxyl or amino group;
R2 is an unsubstituted aryl or aryl substitited as stated above, in particular hydroxyphenyl or C1-C4-alkoxyphenyl; and
R3 is a hydrogen atom.
The novel compounds are prepared from dithionite salts. Advantageously, a salt having a cation which is also desired in the sulfinic acid compounds is used. The dithionite salts are reacted by preparing those compounds in which R2 is an unsubstituted or substituted aryl radical and R3 is a hydrogen atom, with the corresponding aromatic aldehyde. This reaction can be illustrated using sodium dithionite and 2-hydroxybenzaldehyde as an example by the following reaction equation: 
All other compounds of the formula M are prepared by reacting the dithionite salts with the corresponding 1,2-dicarbonyl compound or a sulfonic acid equivalent thereof. The 1,2-dicarbonyl compound used is, in particular, glyoxylic acid or the corresponding keto compounds and their esters. The reaction can be illustrated using sodium dithionite and glyoxylic acid as an example by the reaction equation below: 
The reaction is generally carried out in an aqueous medium in the presence of a base. The aqueous medium may also include water-soluble organic solvents, such as methanol, ethanol, isopropanol, etc. Bases which may be used are, in particular, alkali metal hydroxides and alkaline earth metal hydroxides. The reaction is generally carried out at ambient temperature; heating of the reaction mixture is generally not required because the reaction is exothermic. The desired product generally precipitates out from the reaction mixture or can be precipitated out by adding polar, water-soluble organic solvents, such as methanol, ethanol, isopropanol, acetone, etc. The resulting product is in the form of the salt which can, if desired, be converted into the free sulfinic acid by acidification or treatment with an acidic ion exchanger.
Furthermore, the product is generally produced in a mixture with the corresponding metal sulfite. In many cases, the mixture also contains the corresponding sulfonic acid and water of crystallization. The novel compounds can be separated off from the accompanying constituents in the usual manner, for example by recrystallization from water or aqueous alcohol.
For use in practice, it is not necessary to separate off the accompanying constituents. On the contrary, it has been found that the action of the novel compounds is even increased by these accompanying constituents. The invention thus also provides the corresponding mixtures with the constituents mentioned. For this purpose, the metal sulfite may be present in an amount up to 40% and the sulfonic acid in an amount up to 60%. The water content may be up to 30%.
The novel compounds are reducing agents whose reducing action is comparable with that of formaldehyde sulfoxylate. However, they have the advantage of not eliminating formaldehyde before, during and after use. The novel compounds are thus preferentially used in those fields where the evolution of formaldehyde is undesired. For example, they can be used as reducing agents in textile printing, in particular in textile discharge printing, in textile bleaching or vat dyeing, or as reducing agents for bleaching minerals, such as kaolin etc., and fibers, for example cellulose fibers. They are preferably used, however, as cocatalyst in emulsion polymerization together with peroxidic initiators in order to allow the polymerization to be carried out at a lower temperature. For this purpose, the sulfinic acids may, if desired, be also used together with oxidizable metal ions, such as Fe2+, Mn2+ etc. These metal ions are then advantageously used as counterions to the sulfinic acid compounds, i.e. M=Fe2+, Mn2+ etc.
For use, the novel compounds are generally formulated together with customary additives and auxiliaries. There is no particular limitation in this respect, only that reducing compounds must not be used.
The examples below illustrate the invention without limiting it. The purity figures given in the examples refer to the product which contains water of crystallization which is produced, i.e. the purity is significantly higher when the content of water of crystallization is taken into consideration.